A Mid-IR Pyrometer Calibrated with High-Temperature Fixed Points for Improved Scale Realization to 2,500°C

Glass surface temperature can be measured using a radiation thermometer operating at a mid-IR wavelength, typically the 3–5μm band, where the glass is opaque. For optical fiber preforms, the temperature measurement requirement may exceed 2,200◦C. Scale realization at national measurement institutes at these temperatures is usually carried out at short wavelengths, typically less than 1μm. The mismatch in wavelength can lead to significant uncertainties when calibrating a radiation thermometer working at 3–5μm. To overcome this, a narrow band 3.95μm radiation thermometer has been built that is designed to be used from 1,000 to 2,500◦C. It is calibrated by measurement of high-temperature metal–carbon eutectic fixed-points. The instrument is based on silicon lenses, with a liquid nitrogen (LN2)-cooled InSb detector, and narrow-band iInterference filter. An anti-reflection coated objective lens/aperture stop focuses onto a field stop giving a 1mm target, then a collimating lens, and glare stop. All parts visible to the detector, other than the target area, are either at LN2 temperature or are part of a temperature-stabilized housing. A relay-operated shutter that blocks the field stop is used to subtract the background. The size-of-source effect of the instrument has been measured. Gold-point measurements have been made to assess the stability. The device has been calibrated using high-temperature fixed points. A three-parameter fit has been applied and the resultant scale compared to an ITS-90 realization.